Carrier catalytic converter for theselective hydrogenation of alkines and dienes

ABSTRACT

A catalyst for the selective hydrogenation of alkynes and dienes in C2-C5 + -olefin mixtures is described.  
     These catalysts contain  
     (a) a metal of the tenth group of the Periodic Table,  
     (b) a metal of the eleventh group of the Periodic Table and  
     (c) if required, a compound of a metal of the first or second group of the Periodic Table,  
     these metals being applied to a support which is selected from the group consisting of silica, titanium dioxide, zirconium oxides, spinels, zinc aluminates, zinc titanates or mixtures of these substances, and the metal of the eleventh group being distributed homogeneously over the cross section of the catalyst particle and the metal of the tenth group being present in the edge layer close to the surface of the catalyst particle. Such a catalyst is prepared by applying the metal of the eleventh group, preferably during the preparation of the support itself, by impregnation with a solution of a suitable metal salt.

[0001] The present invention relates to the area of catalysis. Moreprecisely, the present invention relates to a novel hydrogenationcatalyst which makes it possible selectively to hydrogenate more highlyunsaturated hydrocarbons, such as acetylenes and dienes, in olefinmixtures which were obtained by the crack process. The present inventionfurthermore relates to a process for the preparation of such a catalystand a process for the selective hydrogenation of alkynes and dienes inolefin mixtures with the aid of such a catalyst.

[0002] Olefins are generally produced industrially in crack processes,for example by steam cracking or catalytic cracking by FCC. Specificmineral oil distillates are heated to temperatures of about 900° C., atwhich olefins form from the alkanes present. The crude mixture obtainedis then separated by distillation, the fractions being cut in such a waythat the C2 to C5⁺-olefins are separated from one another. The olefinsobtained are then used in further processing. Under the crackingconditions, however, alkynes (acetylenes) and dienes also form, theamounts of which depend on the process and on the experimentalconditions chosen. However, these alkynes and dienes frequently presentproblems during further processing and storage. This is due on the onehand to a tendency to oligomerize and polymerize. Thus, products whichfrequently have to be removed from the product of further processing maybe formed during the further processing. On the other hand, the alkynesand dienes have a strong tendency to form complexes. This presentsproblems in particular when the olefins are subjected to a catalyticprocess in the further processing step. The alkynes or dienes may thenreact with the catalyst and deactivate it or change the activity, whichof course is undesirable.

[0003] For example, in this C2 cut which contains ethylene, acetylene ispresent as an undesired byproduct. Ethylene is further processedcatalytically in large amounts to give polyethylene. Ethylene used forsuch a polymerization may generally have only an acetylene content ofless than about 1 ppm. The same also applies to the C3 stream, whichalso contains propadiene (allene) and propyne in addition to propene.Propene, too, is further processed catalytically in a process which issimilar to that of ethylene in a large amount to give polypropene. Apropene which can be used for the polymerization may generally containonly less than about 10 ppm of allene and propyne.

[0004] In the other cuts of the crack process, too, products which areundesirable for the purposes of further processing form. Depending onthe integration into the profitability chain, in the C4 cut vinylacetylene, an impurity, is hydrogenated before the butadiene extraction.Alternatively, butadiene can be specifically converted into butene, sucha refinement of the C4 stream being desired. The C5⁺ cut contains cyclicpentenes and pentadiene, which should be converted into products whichpresent no problem, linear C5 building blocks and the unsaturated C5⁺components being obtained.

[0005] A process for removing said byproducts is the selectivehydrogenation of these alkynes and dienes. The impurities are convertedin the further processing into components which present no problems orpreferably into the desired product of the hydrocarbon fraction. Themain problem in such a process is that, on the one hand, the catalystused must have sufficient activity completely to hydrogenate thebyproducts, which indeed are present only in relatively small amountscompared with the olefin, and thus to force the content of impurities tovalues which are tolerable in the further processing. When it isconsidered that in some cases a content of impurities of less than 1 ppmhas to be reached, as is the case for polyethylene, it will be clearthat the catalyst used in the selective hydrogenation must have a veryhigh activity.

[0006] On the other hand, such a catalyst must also have a very highselectivity or, in other words, a low specific activity with respect tothe olefin to be further processed, so that this is not hydrogenated oris hydrogenated only to a very small extent even to the correspondingalkane and is no longer available.

[0007] Furthermore, the catalysts used in the selective hydrogenationsshould also have the property of not catalyzing the oligomerization ofalkynes and dienes. In fact, this reaction results in the formation ofoily residues which accumulate on the catalyst. Deactivation of thecatalyst is the result, which can occur even after less than one month,depending on the amount of byproducts formed.

[0008] According to the process described in the prior art, theselective hydrogenation is generally carried out using catalysts fixedon supports and comprising metals which are generally used inhydrogenations, mainly heterogeneous catalysts of the tenth group of thePeriodic Table, i.e. Ni, Pd and Pt. In most cases, Pd is employed.

[0009] The support used is generally a porous inorganic oxide, forexample silica, aluminosilicate, titanium dioxide, zirconium dioxide,zinc aluminate, zinc titanate, spinels and/or mixtures of such supports;generally, however, alumina or silica is used. Furthermore, promoters orother additives may be present. Processes for the selectivehydrogenation of unsaturated compounds in hydrocarbon streams whichcontain them are known both in the form of liquid-phase hydrogenation ormixed gas/liquid-phase hydrogenation, by the trickle-bed or liquid-phaseprocedure, and in the form of pure gas-phase hydrogenation.

[0010] To be able to achieve the desired selectivity, said catalysts aremodified. It is generally known that the desired increases inselectivity in the case of the abovementioned metals can be frequentlyachieved by adding CO during the hydrogenation. However, this requiresspecial safety measures owing to the toxicity of the CO. In addition,this results in a CO-containing product which, for some further uses,first has to be purified to remove CO.

[0011] The prior art contains a large number of references whichdescribe the use of supported palladium catalysts, which were modifiedby addition of promoters, in selective hydrogenations of alkynes anddienes in hydrocarbon streams. In connection with the present invention,the following publications in which the use of alumina as supportmaterial is disclosed are particularly relevant.

[0012] EP-A-0 064 301 describes a catalyst for the selectivehydrogenation of acetylene, which consists of Pd which was modified withAg; the support used is α-alumina. The Pd content is from 0.01 to 0.025%by weight and Ag is present in an amount which is from 2 to 10 timesthat of Pd. In the prepared catalyst, the silver is distributed over allcatalyst particles while 90% of the palladium are present in an edgezone of 300 μm.

[0013] The two applications EP-A-0 686 615 and EP-A-0 780 155 describe acatalyst for the selective gas-phase hydrogenation of alkynes in the C2or C3 stream. The catalyst is palladium to which a metal of group 11 wasadded. The support material used in each case is alumina. At least 80%of both metals are present in a zone which extends from the edge of thecatalyst particle to a radius which is 80% of the external radius of thecatalyst particle. The palladium content is from 0.01 to 0.5% by weightof the catalyst and the ratio of the metal of group 11 to palladium isfrom 0.05 to 0.4 (686 615) or from 0.4 to 3 (780 155). The preferredmetal of group 11 in both applications is silver.

[0014] The German application with the file reference 198 39 459.4,filed on Aug. 28, 1998, describes a catalyst for selective hydrogenationwhich contains at least one hydrogenation-active metal on an aluminasupport and, in the unused state, exhibits in the X-ray diffractionpattern reflections which correspond to the following interplanarspacings (in 10⁻¹⁰ m): 4.52, 2.85, 2.73, 2.44, 2.31, 2.26, 2.02, 1.91,1.80, 1.54, 1.51, 1.49, 1.45 and 1.39, in each case with specificrelative intensities. In a preferred embodiment, thehydrogenation-active metal is platinum and/or palladium, which is dopedwith copper and/or silver.

[0015] The German application with the file reference 198 40 373.9,filed on Sep. 3, 1998, discloses a process in which unsaturatedcompounds in hydrocarbon streams are hydrogenated over a catalyst whichcontains at least one metal of the tenth group of the Periodic Table ofthe Elements and at least one metal of the eleventh group of thePeriodic Table of the Elements on an alumina support, the metal or themetals of the tenth group being concentrated essentially in an edgelayer close to the surface of the catalyst particle, the metal or themetals of the eleventh group being present distributed essentiallyuniformly over the volume of the catalyst particle, and the weight ratioof the metal or of the metals of the eleventh group to the metal or themetals of the tenth group being not more than 1.95.

[0016] All abovementioned publications disclose catalysts in whichalumina is used as the support. There are only a few references whichdisclose selective hydrogenation catalysts which are applied to asupport other than alumina.

[0017] Thus, DE-A-2 156 544 discloses a process for obtaining ethyleneby selective catalytic gas-phase hydrogenation of acetylene in the C2cut of the olefin preparation, a palladium catalyst which is applied toa silica support being used. The catalyst is modified by zinc.

[0018] However, this process has the disadvantage that the oligomerformation occurs to an extent which is too high for present-dayrequirements. Furthermore, the selectivity too is frequentlyinsufficient and the addition of CO proves necessary.

[0019] EP-A-0 764 463 describes a selective hydrogenation catalyst whichcomprises palladium which was modified with a promoter metal of groups 1and 2 of the Periodic Table. Here too, the catalyst is applied to asupport based on silica.

[0020] Here too, the formation of oligomers and hence a reduction of thetime-on-stream of the catalyst are frequently observed.

[0021] DE-P-31 19 850 describes a process which involves the selectivehydrogenation of butadiene in a but-1-ene-containing C4 fraction. Thecatalyst used is applied to alumina or silica having a specific surfacearea of from 10 to 200 m²/g, and the catalyst consists of a mixture ofpalladium and silver or compounds of these metals. The palladium contentis from 0.05 to 5% by weight and the silver content is from 0.05 to 1%by weight. However, the catalyst described in this reference is suitableonly for the selective hydrogenation of butadiene in C4 streams.

[0022] The German application with the file reference 198 40 372.0,filed on Sep. 3, 1998, describes a catalyst which contains, in itsactive material, from 0.05 to 1.0% by weight of at least one metal or acompound of a metal of the tenth group of the Periodic Table of theElements and from 0.05 to 1.0% by weight of at least one metal or acompound of a metal of the eleventh group of the Periodic Table of theElements, the weight ratio of the metal of the eleventh group present tothe metal of the tenth group present being from 0.95 to 1.05, and whichcontains, as a support, a silica-containing catalyst support having aBET surface area of from 2 to 400 m²/g, at least 20% of the total porevolume of the catalyst being present in pores having a diameter above100 nanometers. The catalyst is used for removing alkynes, dienes and/ormonounsaturated hydrocarbons from material streams.

[0023] It may be said that it has not been possible so far to provide aselective hydrogenation catalyst for alkynes and dienes which is appliedto a support material other than alumina, for example silica, and whichhas the same efficiency as catalysts applied to alumina as supportmaterial.

[0024] It is an object of the present invention to provide a catalystfor the selective hydrogenation of alkynes and dienes in C2-C5⁺-olefinmixtures which is applied to a support other than alumina, but which isto have the same efficiency as those catalysts in which alumina was usedas the support. Furthermore, such a catalyst should be very simple toprepare. Preferably, the catalyst should also be lighter than thosecatalysts in which alumina was used as the support material.

[0025] We have found that this object is achieved by a catalyst for theselective hydrogenation of alkynes and dienes in C2-C5⁺-olefin mixtures,the catalyst containing

[0026] (a) a metal of the tenth group of the Periodic Table,

[0027] (b) a metal of the eleventh group of the Periodic Table and

[0028] (c) if required, a compound of a metal of the first or secondgroup of the Periodic Table,

[0029] and these metals being applied to a support which is selectedfrom the group consisting of silica, titanium dioxide, zirconium oxides,spinels, zinc aluminate, zinc titanate or mixtures thereof, and themetal of the eleventh group being distributed homogeneously over thecross section of the catalyst particle and the metal of the tenth groupbeing concentrated in an edge layer close to the surface of the catalystparticle.

[0030] We have found that this object is furthermore achieved by aprocess for the preparation of such a catalyst, wherein the metal of theeleventh group is first homogeneously applied to the support and thenthe metal of the tenth group is applied. Preferably, the metal of theeleventh group is incorporated before the molding of the support, andthe metal of the tenth group is preferably applied by impregnation witha solution of a salt of the respective metal.

[0031] Such a catalyst can advantageously be used in selectivehydrogenations of alkynes and dienes in C2-C5⁺-olefin mixtures. Inconnection with the present invention, olefin mixtures are preferablyunderstood as meaning hydrocarbon streams, i.e. the products which areobtained on cracking mineral oil distillates or natural gas and whichlargely contain olefins. The novel process can however also be used forthe selective hydrogenation of alkynes and dienes in olefin mixtureswhich were obtained in other processes known to those skilled in theart.

[0032] It has surprisingly been found that the selective application ofthe metal of the eleventh group and of the metal of the tenth groupgives a selective hydrogenation catalyst for alkynes and dienes, whichis applied to a support material which is not alumina. However, thecatalyst obtained is just as efficient as those which are applied to analumina support.

[0033] Suitable support material in the context of the present inventionis in particular silica, with which it was possible to achieve the bestresults. Silica has the advantage of having a substantially lowerspecific gravity than alumina. Supported catalysts having a low bulkdensity are generally more economical than catalysts having a high bulkdensity. However, other support materials are suitable for use in anovel hydrogenation catalyst. These are, for example, titanium dioxide,zirconium oxides, zinc aluminate, zinc titanate or mixtures of saidmaterials.

[0034] The support materials used in the present invention have a BETsurface area of from 20 to 400, preferably from 100 to 160, m²/g and apore volume of from 0.1 to 1.5, preferably from 0.7 to 1.2, ml/g.

[0035] The metal of the tenth group is present in amounts of from 0.005to 1, preferably from 0.02 to 0.6, % by weight, based on the total mass,in the novel catalyst. It has been found that, among the metals of thisgroup, i.e. nickel, palladium and platinum, the best results areachieved using palladium. The use of palladium is thus preferred.

[0036] The metal of the tenth group accumulates essentially in an edgelayer close to the surface of the support. In general, more than 80,preferably more than 90, particularly preferably more than 95, % byweight of the metal or of the metals are contained in a layer which hasa thickness of not more than 0.6 mm and is bounded by the geometricsurface of the catalyst particle. Preferably, this layer is not morethan 0.45 mm thick.

[0037] An important element of the catalyst according to the presentinvention is the promoter metal, which is a metal from the eleventhgroup of the Periodic Table, i.e. copper, silver or gold. The additionof this metal and its specific arrangement in the hydrogenation catalystaccording to the present invention permit the selective hydrogenation ofalkynes and dienes with a high activity and selectivity. At the sametime, the tendency to form oligomers and hence the resulting catalystdeactivation are reduced.

[0038] The metal of the eleventh group is present in amounts of from0.005 to 1, preferably from 0.05 to 0.6, % by weight, based on the totalmass, in the novel catalyst. The ratio of metal of the eleventh group tometal of the tenth group is from 0.01 to 100, based on the metal of thetenth group. Preferably, this ratio is in a range from 0.5 to 30,particularly preferably from 1.5 to 20, in which the best results wereachievable. It is furthermore preferred if the metal of the eleventhgroup is silver.

[0039] The catalyst according to the present invention may have acomposition such that in each case only one metal of the eleventh groupand one metal of the tenth group or compounds of these metals arepresent. However, it is also possible for two or more metals of theeleventh group and of the tenth group or compounds thereof to be presentindependently of one another in the catalyst.

[0040] It is particularly preferred if the catalyst according to thepresent invention contains palladium and silver.

[0041] The metal of the eleventh group is distributed over the entirecross section of the catalyst particle in the novel catalyst.

[0042] This can be achieved by methods known to those skilled in theart, such as the incipient wetness method, by impregnating the moldedsupport. Preferably, the metal of the eleventh group is applied beforethe support is molded. For this purpose too, it is possible to use thosemethods known to those skilled in the art, such as

[0043] impregnation of the precipitated support material with the metalof the eleventh group or a compound thereof,

[0044] coprecipitation of the support compound and of the metal of theeleventh group or of a compound thereof,

[0045] mixing of the support material with the metal of the eleventhgroup or a compound thereof in the dry or moist state,

[0046] vapor deposition of the metal of the eleventh group onto thesupport material.

[0047] Thus, a support which contains the metal of the eleventh grouphomogeneously distributed over the support cross section is initiallyobtained.

[0048] In every case, the metal of the eleventh group is applied in astep which is carried out before the application of the metal of thetenth group. This metal of the tenth group can also be fixed on thesupport by the conventional measures known to those skilled in the art.Here too, however, it is once again preferred if this application iseffected by impregnation with a solution of a suitable salt of therespective metal. This is preferably carried out in such a way that thesolution is virtually completely absorbed by the pore volume of thesupport (incipient wetness method). However, the absorptivity of thesupport for the impregnating solution need not be fully exhausted, andthe impregnating solution can therefore be used in an amount of lessthan 100%, for example not more than 95% by volume, not more than 90% byvolume or not more than 85% by volume of the liquid volume absorbed bythe support to be impregnated. The concentration of the salts in thesolution is such that, after impregnation and conversion of theimpregnated support into the finished catalyst, the components to beprecipitated are present in the desired concentration on the catalyst.The salts are chosen so that they do not leave behind any residues whichcould present problems during the preparation of the catalyst or itssubsequent use. In general, nitrates or ammonium salts are used.

[0049] It has been found that the novel desired distribution of themetal of the tenth group is achieved by the initially effected,homogeneous application of the metal of the eleventh group.

[0050] The novel catalysts may also contain further promoter metalswhich are selected from the first and second groups of the PeriodicTable. Sodium, potassium, calcium and barium are preferably used. Theapplication is effected by suitable methods known to those skilled inthe art, for example by impregnation, simultaneously with theapplication of the metal of the tenth and of the eleventh group andindependently of the chosen sequence of application.

[0051] After the application of the metals, the crude catalysts obtainedare dried and calcined at the conventional temperatures, it beingpossible to carry this out in a single step or two separate steps. Thedrying is effected at from 50 to 250° C., preferably from 70 to 100° C.The calcination is carried out at from 250 to 700° C., preferably atfrom 300 to 650° C., it being possible to use, for example, rotatingtubes, belt calciners or muffle furnaces. The moldings have the usualshape, for example extrudates, spheres, rings or pellets, and areprepared by, for example, pelleting or extrusion of the supports.

[0052] The novel catalysts are suitable for selective hydrogenation ofgenerally all alkynes and dienes of 2 to 5 carbon atoms in mixtures ofthese with olefins, generally in hydrocarbon streams obtained oncracking. The hydrogenation can be carried out in the gas phase and inthe liquid phase, analogously to known hydrogenation processes underheterogeneous catalysis. The hydrogenation can be carried out as a puregas-phase process and also as a gas/liquid-phase process. Theseprocesses are known to those skilled in the art. The reactionparameters, for example hydrocarbon throughput, temperature andpressure, are chosen analogously to the known processes.

[0053] The amount of hydrogen used is from 0.8 to 5, preferably from0.95 to 2, times the amount required stoichiometrically for completereaction.

[0054] Examples of hydrogenation processes in which the novel catalystcan be used are mentioned below

[0055] selective hydrogenation of acetylene in C2 streams to giveethylene (referred to below as process A)

[0056] selective hydrogenation of propyne and/or propadiene in C3streams to give propylene (process B)

[0057] selective hydrogenation of 1-butyne, 2-butyne, 1,2-butadieneand/or vinylacetylene in C4 streams to give 1,3-butadiene, 1-butene,cis- and/or trans-2-butene (process C),

[0058] selective hydrogenation of 1-butyne, 2-butyne, 1,2-butadiene,1,3-butadiene and/or vinylacetylene in C4 streams to give 1-butene, cis-and/or trans-2-butene in the case of butadiene-rich C4 streams (crude C4cut) or low-butadiene C4 streams (raffinate I)(process D)

[0059] selective hydrogenation of unsaturated compounds and/orunsaturated substituents of aromatic compounds in C5⁺ streams to givemore highly saturated compounds and/or aromatic compounds with morehighly saturated substituents (process E).

[0060] Process A is usually carried out as a one-stage or multistagegas-phase process with a space velocity of the gaseous C2 stream of from500 to 10,000 m³/m³.h, based on the catalyst volume, at from 0 to 250°C. and from 0.01 to 50 bar.

[0061] Process B is usually carried out as a one-stage or multistagegas-phase process with a space velocity of the gaseous C3 stream of from500 to 10,000 m³/m³.h, based on the catalyst volume, or as agas/liquid-phase process with a space velocity of the liquid C3 streamof from 1 to 50 m³/m³ -h, based on the catalyst volume, at from 0 to180° C. and from 0.01 to 50 bar.

[0062] Process C is usually carried out as a gas/liquid-phase processwith a space velocity of the liquid C4 stream of from 1 to 50 m³/m³.h,based on the catalyst volume, at from 0 to 180° C. and from 2 to 50 bar.Process C can be used, for example, as a selective front-endvinylacetylene hydrogenation before a butadiene extraction.

[0063] Process D is usually carried out as a one-stage or two-stagegas/liquid-phase process with a space velocity of the C4 liquid streamof from 0.1 to 60, preferably from 1 to 50, m³/m³.h, based on thecatalyst volume, at a reactor inlet temperature of from 20 to 90° C.,preferably from 20 to 70, °C. and from 5 to 50, preferably from 10 to30, bar. For example, the process is carried out in two stages, thebutadiene content, which in typical C4 streams from steam crackers isfrom 20 to 80% by weight, based on the total stream, being reduced inthe first stage to a content of from 0.1 to 20% by weight and in thesecond stage to the desired residual content of from a few ppm by weightto about 1% by weight. It is also possible to distribute the totalreaction over more than two reactors, for example three or four. Theindividual reaction stages can be operated with partial recycling of thehydrocarbon stream, the reflux ratio usually being from 0 to 30. Oncarrying out process D, isobutene is retained essentially unchanged andcan be separated from the C4 stream by known methods before or afterprocess D is carried out. Process D can be used, for example, as abutadiene hydrogenation in the C4 stream (if butadiene is not to berecovered as the desired product) or as a selective tail-endvinylacetylene hydrogenation after the butadiene extraction from the C4stream.

[0064] Process E is preferably carried out as a gas/liquid-phase processwith a space velocity of the liquid C5+ stream of from 0.5 to 30m³/m³.h, based on the catalyst volume, at from 0 to 180° C. and from 2to 50 bar. Process E can be used, for example, for the selectivehydrogenation of pyrolysis gasoline, for the selective hydrogenation ofolefins in reformate streams or coke furnace condensates and for thehydrogenation of styrene to ethylbenzene.

[0065] By adding metals of the eleventh group, the support ispreconditioned in the novel catalysts in such a way that the formationof oligomers during the hydrogenation is substantially reduced, incontrast to other hydrogenation catalysts applied to the same supports.The time-on-stream of the catalyst thus increases substantially.

[0066] Furthermore, the addition of CO as a selectivity-controllingagent, which is still frequently required, is also no longer necessary.In the novel catalysts, the hydrogenation-active metal of the tenthgroup can be provided even with large excesses of promoter metal of theeleventh group without a loss of activity during hydrogenation beingobserved.

[0067] The examples which follow illustrate the present invention.

EXAMPLE 1 (Novel Catalyst A)

[0068] A novel catalyst was prepared by preparing a silica support inextrudate form (4 mm extrudates, BET surface area from 120 to 140 m²/g,pore volume from 0.8 to 0.95 ml/g) in such a way that silver in the formof silver nitrate was added in an amount of 0.05% by weight, based onthe amount of SiO₂ used, to the edge mill material during the edgemilling step. After the extrusion to give 4 mm extrudates, the supportwas calcined and was then impregnated with 0.025% by weight, based onthe support material used, of palladium in the form of palladium nitrateat room temperature. The solution volume used corresponded to 90% of thewater absorption of the support. The catalyst was dried at 80° C. andthen calcined at 500° C.

EXAMPLE 2 (Novel Catalyst B)

[0069] A novel catalyst was prepared by preparing a silica support inextrudate form (4 mm extrudates, BET surface area from 120 to 140 m²/g,pore volume from 0.8 to 0.95 ml/g) in such a way that silver in the formof silver nitrate was added in an amount of 0.2% by weight, based on theamount of SiO₂ used, to the edge mill material during the edge millingstep. After the extrusion to give 4 mm extrudates, the support wascalcined and was then impregnated with 0.06% by weight, based on thesupport material used, of palladium in the form of palladium nitrate atroom temperature. The solution volume used corresponded to 90% of thewater absorption of the support. The catalyst was dried at 80° C. andthen calcined at 500° C.

Comparative Example 1 (Comparative Catalyst C)

[0070] A comparative catalyst was prepared by impregnating an aluminasupport in extrudate form, having a BET surface area of 8 m²/g, with anitric acid-containing, aqueous solution of 0.045% by weight, based onthe support material used, of silver in the form of silver nitrate andof 0.025% by weight, based on the support material used, of palladium inthe form of palladium nitrate at room temperature (integral Ag/Pdratio=6.7:1). The solution volume used corresponded to 90% of the waterabsorption of the support. The catalyst was dried at 80° C. and thencalcined at 400° C.

Comparative Example 2 (Comparative Catalyst D)

[0071] A further comparative catalyst was prepared similarly to Example8 of the application DE 2156544, a catalyst containing 0.025% ofpalladium and 0.01% of zinc on a silica gel support in the form of a 4mm extrudate being obtained.

EXAMPLE 3

[0072] Performance Test of Catalysts A to D

[0073] The catalysts A to D were tested in the selective hydrogenationof acetylene in a C2 stream containing about 1% of alkyne at 20 bar anda loading of 3000 l/h and with an 80% H₂ excess, based on thestoichiometric amount. The respective activity, measured with referenceto the reaction temperature, stability, measured with reference to thedeactivation rate of the reaction, and selectivity, measured withrespect to the inclination to oligomer formation, were tested andcompared. Table 1 shows the test results of catalysts A to D. TABLE 1Catalyst A B C D Bulk density 470 470 1100 470 Palladium [% by weight]0.025 0.06 0.025 0.025 Ag/Pd ratio 2 3.3 1.8 — CO addition for not notnot required performance required required required stabilizationReaction temperature 65 50 45 65 [° C.] Deactivation rate of the −0.048−0.002 −0.001 −0.159 reaction [%/h] Oligomer formation [g/h] 0.024 0.0120.014 0.051

[0074] The results obtained show that the addition of silver to thesupport gives improved catalysts with respect to all characteristics (Aand B as compared with D). Furthermore, it is clear that the addition ofsilver to the supports makes it unnecessary to add CO for increasing theselectivity (A and B as compared with D) and that higher Ag/Pd ratioslead to less coking and a slower deactivation rate (A as compared withD). Moreover, a substantial increase in activity can be achieved byincreasing the amount of palladium (A as compared with B and D).

[0075] Compared with catalyst C, catalyst A and in particular catalyst Bare catalysts which are substantially lighter and hence more economicalfor the operator and exhibit very good performance corresponding to theprior art (catalyst C).

EXAMPLE 4

[0076] The novel catalysts E to U were obtained by the following methodand with variation of the individual preparation parameters stated inTable 2:

[0077] A silver-doped SiO₂ support in the form of 4 mm extrudates wasprepared by precipitating silica at pH 6 from an ammoniacal sodiumwaterglass solution by adding sulfuric acid, filtering off said silicafrom the aqueous phase, eliminating its conductivity by washing withwater and spraying it to give a powder (bulk density, varying accordingto production, of from 350 to 450 g/l, water content about 25%). Thespray-dried powder obtained was kneaded (i.e. treated in an edge mill)with water and with the addition of a defined amount, based on the solidmaterial used, of silver in the form of aqueous silver nitrate solutionfor one hour to give an extrudable material. The material treated in theedge mill was molded to give 4 mm extrudates. The extrudates were driedat 120° C. and calcined at a defined temperature. A product having a BETsurface area of from 110 to 160 m²/g and a bulk density of from 440 to480 g/l was obtained. The exact surface area values and bulk densitiesare shown in Table 1.

[0078] This support material was then impregnated with an amount,defined on the basis of the support material used, of palladium in theform of aqueous palladium nitrate solution at room temperature withagitation. The solution volume used corresponded to 95% of the waterabsorption of the support. The material obtained was dried withadmission of air at a defined temperature and with agitation and thencalcined for 1 hour at a defined temperature with agitation. The exactpreparation parameters are shown in Table 1.

[0079] Performance Tests of Catalysts E to U

[0080] The catalysts E to U were tested at 1 bar in a straight pass. TheGHSV was 3000 l/h. The stream was composed of 1% by volume of acetylenein ethylene, and 1.8 equivalents, based on the amount of C₂H₂ used, ofH₂ were added. The temperatures required for a 90% conversion of theacetylene and the selectivity values obtained are shown in Table 2.TABLE 2 Catalyst E F G H I J K L M N O P Q R S T U Palladium [% byweight] 0.025 0.025 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.060.06 0.06 0.04 0.065 0.025 Silver [% by weight] 0.15 0.2 0.2 0.3 0.2 0.20.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Ag/Pd ratio 6 8 3.33 5 3.333.33 3.33 3.33 3.33 3.33 3.33 3.33 3.33 3.33 5 3.1 8 Support propertiesCalcination temperature 850 850 850 850 850 850 850 850 850 850 850 850850 850 850 850 940 [° C.] Bulk density [g/l] 470 430 430 400 470 470470 470 470 470 470 470 470 470 470 470 470 BET surface area [m²/g] 144124 124 112 140 140 159 159 159 159 159 159 159 159 159 159 97 Waterabsorption [ml/g] 1.06 0.98 0.98 0.82 0.89 0.89 0.87 0.87 0.87 0.87 0.870.87 0.87 0.87 0.87 0.87 0.86 Catalyst drying conditions Temperature [°C.] 80 80 80 200 200 200 100 200 300 200 200 200 200 200 200 200 80 Time[h] 2 2 2 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 2 Moiststanding time until 0.5 0.5 0.5 0.5 0.5 188 0.5 0.5 0.5 0.5 0.5 0.5 0.50.5 0.5 0.5 0.5 drying [h] Catalyst calcination conditions Temperature[° C.] 360 500 500 500 500 500 500 500 500 300 350 400 450 500 500 500500 Performance test Temperature for 90% 119 104 73 81 64 54 61 63 57 8169 62 67 63 71 68 103 conversion [° C.] Selectivity at 90% conver- 63 4019 41 28 38 42 27 31 40 38 18 35 26 15 6 49 sion [%]

We claim:
 1. A catalyst for the selective hydrogenation of alkynes anddienes in C2-C5⁺-olefin mixtures, the catalyst containing (a) a metal ofthe tenth group of the Periodic Table, (b) a metal of the eleventh groupof the Periodic Table and (c) if required, a compound of a metal of thefirst or second group of the Periodic Table, and these metals beingapplied to a silica support, and the metal of the eleventh group beingdistributed homogeneously over the crosssection of the catalyst particleand the metal of the tenth group being present in an edge layer close tothe surface of the catalyst particle.
 2. A catalyst as claimed in claim1, wherein the metal of the tenth group is palladium.
 3. A catalyst asclaimed in claim 1 or 2, wherein the metal of the eleventh group issilver.
 4. A catalyst as claimed in any of claims 1 to 3, wherein themetal of the tenth group is present in an amount of from 0.005 to 1,preferably from 0.02 to 0.6, % by weight, based on the total mass.
 5. Acatalyst as claimed in any of claims 1 to 4, wherein the metal of theeleventh group is present in an amount of from 0.005 to 1, preferablyfrom 0.05 to 0.6, % by weight and the ratio of metal of the eleventhgroup to the metal of the tenth group ranges from 0.01 to 100,preferably from 0.5 to 30, particularly preferably from 1.5 to
 20. 6. Aprocess for the preparation of a catalyst as claimed in any of claims 1to 5, wherein first the metal of the eleventh group is applied to thesupport in such a way that it is homogeneously distributed and then themetal of the tenth group is applied.
 7. A process as claimed in claim 6,wherein the application of the metal of the eleventh group is effectedbefore or after the molding of the support, preferably before themolding of the support.
 8. A process as claimed in claim 7, wherein themetal of the eleventh group is introduced by kneading into the supportmaterial during the edge milling step.
 9. A process for the selectivehydrogenation of alkynes and dienes in C2-C5⁺-olefin mixtures,preferably in C2- or C3-streams of the olefin preparation by cracking,wherein a catalyst as claimed in any of claims 1 to 5 is used in thehydrogenation.
 10. A process as claimed in claim 9, wherein thehydrogenation is to be carried out at from 0 to 250° C. and from 0.01 to50 bar.